The primary objective of the proposed research involves determination of an efficient genetic analysis which will allow a relatively complete partitioning of sources of variation in complex phenotypes associated with thermoregulation in small mammals. Although most characters of importance to animals (including humans) belong in this category, there have been few genetic analyses included in biological investigations of quantitatively varying phenotypes, and fewer which represent attempts to evaluate both efficiency and validity of the procedures. Genetic analysis of multiple traits associated with thermoregulation will emphasize diallel crosses, one involving 4 inbred strains of mice and the other consisting of the 6 lines of replicated bidirectional selection experiment (with controls) for nest-building. One aim will be to quantify the relative amounts of additive genetic and dominance variance, and results will be compared with previous analyses which utilized correlated response to selection and parent-offspring comparisons. A secondary objective is to begin analysis of the mechanisms underlying physiological differences which have resulted from divergent selection for behavioral thermoregulation (nest-building). Physiological investigation of high and low nesting mice will include metabolic measurements in the cold, measurement of insulative capacity and relative amounts of shivering and non-shivering thermogenesis, micrometabolic studies of brown adipose tissue, possibly including studies of enzyme kinetics.